(phylum Mollusca, class Bivalvia)
An order of unequilateral, and generally equivalved, marine and non-marine bivalves whose shell shape and size is very variable and whose ornamentation is usually simple. The musculature is isomyarian to sub-isomyarian. The ligament is external and opisthodetic. The dentition is quite variable, forms having one or two teeth, some pseudotaxodont and some edentulous. They first appeared in the Late Devonian.
Subjects: Zoology and Animal Sciences.